It
is both. But that answer requires looking more deeply at the meanings of the
words "theory" and "fact."

In
everyday usage, "theory" often refers to a hunch or a speculation. When people
say, "I have a theory about why that happened," they are often drawing a
conclusion based on fragmentary or inconclusive evidence.

The
formal scientific definition of theory is quite different from the everyday
meaning of the word. It refers to a comprehensive explanation of some aspect of
nature that is supported by a vast body of evidence.

Many
scientific theories are so well established that no new evidence is likely to
alter them substantially. For example, no new evidence will demonstrate that
the Earth does not orbit around the Sun (heliocentric theory), or that living
things are not made of cells (cell theory), that matter is not composed of
atoms, or that the surface of the Earth is not divided into solid plates that
have moved over geological timescales (the theory of plate tectonics). Like
these other foundational scientific theories, the theory of evolution is
supported by so many observations and confirming experiments that scientists
are confident that the basic components of the theory will not be overturned by
new evidence. However, like all scientific theories, the theory of evolution is
subject to continuing refinement as new areas of science emerge or as new
technologies enable observations and experiments that were not possible
previously.

One
of the most useful properties of scientific theories is that they can be used
to make predictions about natural events or phenomena that have not yet been
observed. For example, the theory of gravitation predicted the behavior of
objects on the Moon and other planets long before the activities of spacecraft
and astronauts confirmed them. The evolutionary biologists who discovered Tiktaalik predicted that they
would find fossils intermediate between fish and limbed terrestrial animals in
sediments that were about 375 million years old. Their discovery confirmed the
prediction made on the basis of evolutionary theory. In turn, confirmation of a
prediction increases confidence in that theory.

In
science, a "fact" typically refers to an observation, measurement, or other
form of evidence that can be expected to occur the same way under similar
circumstances. However, scientists also use the term "fact" to refer to a
scientific explanation that has been tested and confirmed so many times that
there is no longer a compelling reason to keep testing it or looking for
additional examples. In that respect, the past and continuing occurrence of
evolution is a scientific fact. Because the evidence supporting it is so
strong, scientists no longer question whether biological evolution has occurred
and is continuing to occur. Instead, they investigate the mechanisms of
evolution, how rapidly evolution can take place, and related questions.